Influence of plasma parameters on the properties of ultrathin Al₂O₃ films prepared by plasma enhanced atomic layer deposition below 100 °C for moisture barrier applications

The dependence of the Al₂O₃ growth-per-cycle (GPC) on Si substrates as function of the plasma power is shown in Figs. 2(a) and 2(b) for O₂ concentrations of 9 and 20%, respectively. The plasma power was varied between 50 and 300 W for three plasma exposure durations of 1, 3, and 6 s. As can be seen in Figs. 2(a) and 2(b), the lowest GPC of 1.07 Å/cycle is obtained with an O₂ concentration of 9%, a power of 50 W and an exposure time of 1 s. The highest GPC of 1.57 Å/cycle is also obtained for a plasma exposure time of 1 s by using power values of 300 and 180 W and O₂ concentrations of 9 and 20%, respectively. Still for an exposure time of 1 s, when the O₂ concentration is set to 9%, the GPC increases with an increase in plasma power to reach the highest value. A similar trend is seen for plasma powers between 50 and 180 W when the O₂ concentration is changed to 20%. However, for a power of 300 W, while no significant change in GPC was observed, obvious thickness deviations were detected. For plasma exposure durations of 3 and 6 s, the GPC is found to decrease with an increase in plasma power for an O₂ concentration of 20%, however it remains rather constant (within the error margins) in the whole plasma power range for an O₂ concentration of 9%. Note that (a) for both 9 and 20% O₂ concentrations, the highest GPC is found to decrease with increasing plasma exposure time above 1 s and (b) for an O₂ concentration of 20%, similar GPC was obtained for exposure durations of 1 and 6 s and a power of 50 W.

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Figures 3(a) and 3(b) show the refractive index dependence of Al₂O₃ films on plasma power measured by ellipsometry for plasma exposure durations of 1, 3, and 6 s as well. As a general observation, the refractive index is found to increase with increasing exposure time. While the highest refractive indices are obtained with a plasma exposure time of 6 s, the lowest one (n ∼ 1.60) is obtained for samples grown with an exposure time of 1 s and a power of 50 W. For a plasma exposure time of 1 s, a rather constant value (within the error margins) is measured above 100 and 180 W for O₂ concentrations of 20 and 9%, respectively. Further increase of the plasma exposure time to 3 s shows a similar behavior for both 9 and 20% O₂ concentrations, i.e., nearly constant refractive index above 100 W. However, for 6 s although a similar trend is seen for an O₂ concentration of 20%, the refractive index remains constant in the whole power range (from 50 to 300 W) for an O₂ concentration of 9%.

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To study the effect of plasma power on the chemical composition of Al₂O₃ films, ATR-FTIR spectroscopy was carried out on samples grown on Si substrates with power values of 50, 100, and 180 W. The ATR-FTIR transmission spectra are shown in Fig. 4. As a general observation, the Al–O bond band fingerprint of amorphous Al₂O₃ is seen between 525 and 1000 cm⁻¹ for all samples.^22,23) Additionally, all spectra have a strong Si–O mode band at 1107 cm⁻¹,²³⁾ which hints to the presence of an interfacial silicon oxide layer. The broad band features in the 3800–3200 cm⁻¹ region can be assigned to the –OH groups.^22,24) These bands exhibit a negative transmission, i.e., peak-like-shapes instead of valleys around 3700 cm⁻¹ (see inset of Fig. 4). This is due to the automatic transmission spectrum ratioing scheme in which the sample spectrum is divided by the silicon background spectrum; the latter having a stronger relative presence of –OH groups.²⁵⁾ Still, the relative differences of the spectra can reveal information on films as the used background spectrum and measurement conditions were similar for all samples. Note that the spectrum of the sample grown with a power of 50 W is unique as it shows (a) a more pronounced intensity of the –OH band compared to the other samples, (b) band overlapping of the bending overtone, symmetric and asymmetric stretching vibrations of the C–H groups in the 2980–2870 cm⁻¹ range,²⁴⁾ (c) a band located at 1239 cm⁻¹ which is assigned to deformation vibrations of methyl^24,26) and supports the significant presence of –CH_x species and (d) pronounced band forms in the 1800–1350 cm⁻¹ region which are assigned to symmetric and asymmetric stretching modes of OCO molecule structures.²⁴⁾

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XRR measurements and simulations were carried out to evaluate the film density and surface roughness of the PEALD Al₂O₃ samples. In Fig. 5, the film density and roughness as function of plasma power are shown for Al₂O₃ layers grown with a plasma exposure time of 1 s and an O₂ concentration of 20%. Within measurement errors, density values are constant (∼2.85 g·cm⁻³) for plasma powers of 100, 180, and 300 W, while the 50 W process leads to films with a lower density of 2.6 g·cm⁻³. While a high roughness of 1.22 nm was obtained for a plasma power of 50 W, a sub-nanometer roughness was measured for samples grown with higher powers; the lowest value being 0.6 nm at 100 W. The analyses also reveal the presence of a ∼1.3 nm interfacial layer of silicon oxide in all measured Al₂O₃ samples except for the sample grown with a power of 300 W, for which the silicon oxide layer has a thickness of about 0.7 nm.

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Moisture permeation barrier properties of 4-nm-thick PEALD Al₂O₃ films grown on PEN substrates with a constant O₂ concentration of 20% and varied plasma powers and exposure durations were investigated. As shown in Fig. 6, the uncoated PEN has an initial WVTR of 1.4 g·m⁻²·day⁻¹. The barrier performance was improved over 250 times upon the deposition of the Al₂O₃ thin films. For plasma exposure time below 6 s, samples prepared with 100 and 180 W show similar barrier properties which are better than those of samples grown with 50 W. Power of 50 W and exposure durations of 1 and 3 s led to WVTR of 4.4 × 10⁻² and 1.1 × 10⁻² g·m⁻²·day⁻¹, respectively. An improvement by almost an order of magnitude was observed with further increase of the plasma power leading to a WVTR of ∼7 × 10⁻³ and 5 × 10⁻³ g·m⁻²·day⁻¹ for both 1 and 3 s, respectively. With a further increase of the exposure time to 6 s, while the barrier performance of the sample prepared with a power of 50 W improved leading to WVTR of ∼6 × 10⁻³ g·m⁻²·day⁻¹, samples grown with powers of 100 and 180 W showed a degradation. Table I summarizes WVTR values for polymer substrates coated with PEALD Al₂O₃ films. The best WVTR obtained in this work is compared to results reported by other authors. For comparison purposes, only processes with a temperature window between room temperature and 100 °C were included. While the best WVTR values (in the order of 10⁻⁴ g·m⁻²·day⁻¹) were reported for 100-nm-thick Al₂O₃, WVTR above 10⁻³ g·m⁻²·day⁻¹ were measured for samples below 50 nm. Moreover, these data show that reasonable WVTR values of about 5 × 10⁻³ g·m⁻²·day⁻¹, were achieved even for ultrathin Al₂O₃ barrier with thicknesses under 5 nm.

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Because plasma power, exposure time and O₂ concentration influence the growth behavior of Al₂O₃ thin films, to achieve a saturated growth, an optimization of process parameters is necessary. The lowest GPC, which is obtained with a 9% O₂ concentration, a plasma power of 50 W and an exposure time of 1 s, is most likely caused by the low O₂ concentration, insufficient plasma exposure time and power that lead to a shortage of O radicals and therefore incomplete gas–solid interactions. Consequently, high level carbon impurities are expected to be left in the film and inhibit thereby the film growth. In fact, a strong concentration of C–H and O–C–O group impurities in the sample grown with a power of 50 W was measured by ATR-FTIR even for samples grown with the O₂ concentration of 20%. Moreover, the OCO molecule structure, which is part of carbonate (COO₂–) and formate (HCOO–) group structures, was previously reported to appear in TMA-O₂ plasma Al₂O₃ ALD processes with insufficient plasma time.^22,24,27) This indicates that a higher plasma power, exceeding 50 W, is needed to complete the intermediate surface species removal during the O₂ plasma half-cycle. For a plasma exposure time of 1 s, higher plasma powers (100–300 W) have led to higher growth rates confirming the incomplete surface saturation during the process with a power of 50 W. The high growth rates observed above 50 W are correlated to the reduced concentration of incomplete –OH groups and carbon impurities which can be explained by the surface chemical reactions described in Ref. 27. The refractive index, density and roughness measured for PEALD Al₂O₃ films grown with a plasma exposure time of 1 s and an O₂ concentration of 20% support the above-mentioned growth behavior. The low refractive index, low density and high roughness of the samples grown with a power of 50 W confirm the hypothesis of an incomplete surface reaction and a high concentration of impurities whereas the higher refractive indices, the higher densities and lower roughness measured for samples prepared with powers above 100 W point to a more complete reaction. The increase of the refractive index with decreased impurity levels was also reported elsewhere.⁷⁾ The saturation of the ALD surface reaction often leads to a saturated GPC, which is seen as the highest value of ∼1.57 Å/cycle in this study and correlates here with a refractive index of 1.63. In the case of samples prepared with a power of 300 W, we suspect that the thickness deviations, which were detected by ellipsometry, result from surface etching of the films since a thinner silicon oxide interfacial layer was measured by XRR. The difference in the silicon oxide thickness would then be related to the stronger ion bombardment with the 300 W plasma power causing a minor sputtering effect on the deposited surface.

Unlike for 1 s, for longer plasma exposure durations the GPC is found to either remain constant with an increase of the plasma power as in the case of 9% O₂ concentration or decrease as in the case of 20% concentration. The nearly constant GPC values, which are obtained for an O₂ concentration of 9%, cover the whole plasma power range. This suggests that the plasma pulse length has a more dominant role in the reaction than the plasma power when the reaction time is long enough. The decrease of the GPC that is observed for an O₂ concentration of 20% is assigned to film densification since the refractive index remains above 1.63. Based on our findings and taking into consideration the GPC and the refractive index values, we speculate that the growth of our films takes place through three specific stages. The first stage, in which the film growth is unsaturated, results in a low GPC and a low refractive index. The second stage, in which the growth saturation of the films occurs, leads to an increase of the GPC and the refractive index. Finally, the third stage in which densification of the films dominates, leads to decreased GPC while the refractive index remains high. Thus, for an O₂ concentration of 20% and a power of 50 W, although a similar GPC of 1.38 Å/cycle is obtained for the films grown with an exposure time of 1 and 6 s, their growth behavior can be linked to the unsaturated growth stage and film densification stage, respectively. The latter results from an extended exposure (6 s) to plasma despite the low power (50 W). The suggested growth stages dependence on plasma parameters are summarized in Table II.

The WVTR is a key criterion for describing the moisture permeation barrier properties of thin films, and measurements results show that it is strongly influenced by plasma parameters during the PEALD process. The WVTR dependence on plasma parameters correlates with the suggested growth behavior mentioned above and summarized in Table II, i.e., unsaturated growth stage leads to high WVTR whereas saturated or densification stage results in improved moisture resistance. Nonetheless, for powers ≥100 W and an exposure time of 6 s, where densification is expected to take place, we see a degradation of the moisture barrier. In this case, we speculate that ion bombardment during the longer plasma exposure (6 s) might have resulted in the PEN substrate damage as observed for the Si substrate-coated sample grown with a power of 300 W. Therefore, we conclude that the Al₂O₃ film growth with PEALD for encapsulation applications requires optimum plasma power and exposure time which lead to a high film quality without substrate damage. While good Al₂O₃ permeation barrier properties were reported for thicknesses above a few tens of nanometers,^10,12) the results summarized in Table I show that reasonable moisture permeation barrier properties can also be reached for ultrathin Al₂O₃ films. Starostin et al. demonstrated that excellent encapsulation films for OLED applications can be processed by combining an ultrathin Al₂O₃ film grown by PEALD with a SiO₂ buffer layer.²⁸⁾ However, they also reported that their Al₂O₃ films with a thickness below 5 nm and directly grown on PEN substrates revealed a degradation behavior during the water vapor permeation measurements. Our results show that by choosing optimum plasma parameters of the PEALD process, including an appropriate plasma power and exposure time, comparable properties for moisture barrier applications are reachable below 5 nm. Therefore, such films could potentially be used as moisture barriers in flexible devices.